1. Field of the Invention
The present invention relates to a forged product obtained by hot forging of a steel ingot and a crankshaft obtained using it, particularly to a forged product wherein the dispersion state of inclusions present in steel is controlled.
2. Description of the Related Art
For machinery, a vessel, a generator or the like, a rotary motion part (crankshaft, for example) is used as a member transferring energy of a driving source. The rotary motion part is manufactured using, for example, a forged product obtained by hot forging of a steel ingot manufactured by an ingot-making method, and fatigue properties wherein a fatigue failure hardly occurs even under a severe using environment are required.
With regard to low alloy steel adopted for a crankshaft for a vessel, “Study on fatigue strength properties of high strength crankshaft material”, Journal of the JIME, 2001, Vol. 36, No. 6, p. 385-390, describes that (1) an inclusion is liable to become a starting point of a fatigue failure and such tendency becomes conspicuous as the steel is high-strengthened, (2) as the size of inclusions becomes large, fatigue properties lower, (3) steel material containing elongated inclusions is liable to show anisotropy of fatigue strength, and the like. Also it describes that, in order to improve fatigue properties of a forged product, it is effective to make the shape of inclusions spherical and to make the size small.
Incidentally, in order to manufacture a large-size crankshaft used for a vessel or the like, a large-size ingot is naturally required. When a large-size ingot is manufactured by an ingot-making method, a settled crystal zone is formed in the bottom part of the ingot (that means the end part in the gravity direction). The settled crystal means a crystal nucleus of steel formed on the surface of molten steel (boundary face between heat insulating material and oxidation inhibitor and molten steel) during and after molten steel is poured into a mold, molten from its root by molten steel convection within the mold, physically divided, and settled in the molten steel by density difference with the molten steel. While the crystal nucleus settles in the molten steel, a part of inclusions in the molten steel is caught, and inclusions segregate in the bottom part of the steel ingot. Aggregation of the settled crystals and inclusions thus formed is the settled crystal zone. Formation of the settled crystal zone becomes a cause of deterioration of fatigue properties in the bottom part of the steel ingot. The settled crystal zone does not appear in the steel ingot manufactured by a continuous casting method.
As a technology for reducing and inhibiting accumulation of inclusions in the settled crystal zone, Japanese Published Unexamined Patent Application No. S49-35232 discloses that, in manufacturing killed steel by a bottom-pouring method or top-pouring method, the top surface of the molten steel in the mold is to be covered with an early combustion type high-calorie heat insulating agent during pouring of the molten steel or immediately after pouring of the molten steel.
In Japanese Published Unexamined Patent Application No. S59-178153, it is described that, in order that inclusions do not concentrate and remain in the bottom part of a steel ingot, a top part heat insulating material-cum-antioxidant is to be used, the pouring temperature of the molten steel is to be made in the range of the melting point plus 75-100 DEG. C., and 90% or more of the top part heat insulating material-cum-antioxidant is to be added prior to pouring of the molten steel or within 60% or less of the total pouring time.
As described in Japanese Published Unexamined Patent Application Nos. S49-35232 and S59-178153, in order to inhibit formation of the settled crystal zone, it is effective to heat-insulate the upper part of the steel ingot (which means the end part opposite to the bottom part), however the fatigue strength may possibly vary between the upper part and the bottom part of the steel ingot, and there was a case that a homogenous forged product could not be obtained. Also, when the fatigue strength of an axis position of the steel ingot and the peripheral part apart from the axis position in the radial direction are measured, a difference in the fatigue strength is recognized according to the measuring position.